The invention is directed to a single mode optical waveguide fiber, more particularly to a waveguide fiber in which the total dispersion is maintained at a low value over a selected wavelength range.
Because of the high data rates and the need for long regenerator spacing, the search for high performance optical waveguide fibers designed for long distance, high bit rate telecommunications has intensified. An additional requirement is that the waveguide fiber be compatible with optical amplifiers, which typically show an optimum gain curve in the wavelength range 1530 nm to 1570 nm. Consideration is also given to the potential of expanding the usable wavelength into the L-Band range of about 1570 nm to 1700 nm, more preferably in the range of about 1570 nm to 1625 nm. Another optical waveguide fiber operating wavelength range is the band that extends from about 1250 nm to 1350 nm. Although attenuation in this lower band is greater in comparison to the higher wavelength windows of operation, this lower wavelength band can provide additional information channels that significantly increase overall system capacity.
In cases where waveguide information capacity is increased by means of wavelength division multiplexing (WDM) technology, an additional waveguide fiber property becomes important. For WDM, high bit rate systems, the waveguide should have exceptionally low, but non-zero, total dispersion over the wavelength range of operation, thereby limiting the non-linear dispersion effect of four wave mixing.
Another non-linear effect that can produce unacceptable dispersion in systems having a high power density, i.e., a high power per unit area, is self phase modulation. Self phase modulation may be controlled by designing a waveguide core which has a large effective area, thereby reducing the power density. An alternative approach is to control the sign of the total dispersion of the waveguide so that the total dispersion of the waveguide serves to counteract the dispersion effect of self phase modulation.
A waveguide having a positive dispersion, where positive means shorter wavelength signals travel at higher speed than those of longer wavelength, will produce a dispersion effect opposite that of self phase modulation, thereby substantially eliminating self phase modulation dispersion.
Such a waveguide fiber is disclosed and described in U.S. patent application Ser. No.08/559,954, incorporated herein in its entirety by reference. The present novel profile improves upon the Ser. No.08/559,954 fiber by increasing effective area. In addition the waveguide of this disclosure has a total dispersion over the wavelength window of operation that is everywhere positive and has a lower limit greater than about 2.0 ps/nm-km to further reduce the power penalty due to four wave mixing.
Thus there is a need for an optical waveguide fiber which:
is single mode over at least the wavelength range 1530 nm to 1570 nm, and preferably over a range that extends to the lower wavelength 1250 nm;
has a zero dispersion wavelength outside the range 1530 nm to 1570 nm;
has a positive total dispersion over the wavelength range 1530 nm to 1625 nm which is not less than about 2.0 ps/nm-km;
has low attenuation, less than about 0.25 dB/km, over the range of about 1570 nm to 1625 nm; and
retains the usual high performance waveguide characteristics such as high strength and acceptable resistance to bend induced loss.
The concept of adding structure to the waveguide fiber core by means of core segments, having distinct profiles to provide flexibility in waveguide fiber design, is described fully in U.S. Pat. No. 4,715,679, Bhagavatula. The segmented core concept can be used to achieve unusual combinations of waveguide fiber properties, such as those described herein.
Definitions
The following definitions are in accord with common usage in the art.
The refractive index profile is the relationship between refractive index and waveguide fiber radius. The core refractive index profiles of the invention are described in terms of upper and lower profile boundaries. In addition particular embodiments are described in terms of the relative index xcex94(r)% (defined below) value at a number of radius points. The points chosen fully describe the refractive index profile in each case.
The radii descriptive of the index profiles disclosed herein appear in the drawings.
The effective area is,
Aeff=2xcfx80(∫E2r dr)2/(∫E4r dr),
where the integration limits are 0 to ∞, and E is the electric field associated with the propagated light. An effective diameter, Deff, may be defined as,
Aeff=xcfx80(Deff/2)2.
The initials WDM represent wavelength division multiplexing.
The initials SPM represent self phase modulation, a non-linear optical phenomenon wherein a signal having a power density above a specific power level will travel at a different speed in the waveguide relative to a signal below that power density. SPM causes signal dispersion comparable to that of linear dispersion having a negative sign.
The initials FWM represent four wave mixing, the phenomenon wherein two or more signals in a waveguide interfere to produce signals of different frequencies.
The term, xcex94%, represents a relative measure of refractive index defined by the equation,
xe2x80x83xcex94%=100xc3x97(ni2xe2x88x92nc2)/2ni2,
where ni is the maximum refractive index in region i, unless otherwise specified, and nc is the average refractive index of the cladding region unless otherwise specified.
The term xcex1-profile refers to a refractive index profile, expressed in terms of xcex94(b) %, where b is radius, which follows the equation,
xcex94(b)%=xcex94(bo)(1xe2x88x92[|bxe2x88x92bo|/(b1bo)]xcex1),
where bo is the maximum point of the profile and b1 is the point at which A(b)% is zero and b is in the range bixe2x89xa6b xe2x89xa6bf, where xcex94% is defined above, bi is the initial point of the xcex1-profile, bf is the final point of the xcex1-profile, and xcex1 is an exponent which is a real number.
In a computer model of the profile, in order to bring about a smooth joining of the xcex1-profile with the profile of the adjacent profile segment, the equation is rewritten as;
xcex94(b)%=xcex94(ba)+[xcex94(bo)xe2x88x92xcex94(ba)]{(1xe2x88x92[|bxe2x88x92bo|/(b1xe2x88x92bo)]xcex1},
where ba is the first point of the adjacent segment.
The pin array bend test is used to compare relative resistance of waveguide fibers to bending. To perform this test, attenuation loss is measured when the waveguide fiber is arranged such that no induced bending loss occurs. This waveguide fiber is then woven about the pin array and attenuation again measured. The loss induced by bending is the difference between the two attenuation measurements. The pin array is a set of ten cylindrical pins arranged in a single row and held in a fixed vertical position on a flat surface. The pin spacing is 5 mm, center to center. The pin diameter is 0.67 mm. The waveguide fiber is caused to pass on opposite sides of adjacent pins. During testing, the waveguide fiber is placed under a tension just sufficient to make the waveguide conform to a portion of the periphery of the pins.
Another bend test referenced herein is the lateral load test. In this test a prescribed length of waveguide fiber is placed between two flat plates. A #70 wire mesh is attached to one of the plates. (The market code #70 mesh is descriptive of screen made of wire having a diameter of 0.178 mm. The screen openings are squares of side length 0.185 mm.) A known length of waveguide fiber is sandwiched between the plates and a reference attenuation is measured while the plates are pressed together with a force of 30 newtons. A 70 newton force is then applied to the plates and the increase in attenuation in dB/m is measured. This increase in attenuation is the lateral load attenuation of the waveguide.
The low attenuation, large effective area waveguide fiber disclosed and described herein meets the requirements listed above and, in addition, lends itself to reproducible manufacture. The fiber usually is configured to propagate a single mode over the wavelength range of about 1530 nm to 1625 nm. Index profiles designs for use around the 1310 wavelength window are also disclosed. However the invention includes configuration of core and clad refractive index profiles that propagate more than one mode over all or part of the operating wavelength range. In the case in which more than one mode is propagated, all but the lowest order mode is strongly attenuated in the fiber. Thus the higher order modes disappear within a distance less than 1 km of fiber. Thus, in terms of typical transmission distances, the fiber effectively propagates only a single mode.
A first aspect of the invention is an optical waveguide fiber having a core region and a surrounding clad layer. The clad layer is in contact with the outside surface of the core region. The core region and clad layer are each characterized by respective refractive index profiles. That is, a value of relative index xcex94(r) % is defined for each radius point of the core region and clad layer. In the case of the core region, the radius points are in the range zero, at the core centerline, to ro, the radius drawn from the centerline to the interface of the core region and clad layer. At the centerline, the zero radius point, xcex94(r)%=xcex94o% is in the range of 0.25% to 1%. The core to clad interface radius, ro, is in the range 5.8 xcexcm to 18 xcexcm. The value of xcex94(r)% at the interface radius is zero. At radius points between the centerline and the interface, the relative index is bounded by an upper and a lower refractive index profile curve. The boundary profiles are selected so that the effective area of the waveguide is greater than or equal to 80 xcexcm2 and has attenuation less than 0.20 dB/km, where both of these values are taken at a wavelength of 1550 nm.
In a first embodiment of this aspect of the invention, the respective upper and lower boundary profiles are given as curves AB and CD in FIG. 3.
In a second embodiment of the invention, the respective upper and lower boundary profiles are given as curves EF and GH in FIG. 4.
The properties of waveguide fibers made in accord with the embodiments of the first aspect of the invention are set forth in Tables 1 and 2 below.
In a second aspect of the invention, the refractive index profile of the core region is described in terms of specific values of xcex94(r)% at a sufficient number of points to fully describe the core region profile. In particular, the xcex94(r)% on centerline is in the range 0.8% to 0.9%. The profile shape of core region is an xcex1-profile with xcex1=1 over the radius range zero to 1+/xe2x88x920.2 xcexcm. The final point of xcex1-profile has a relative index percent in the range 0.35% to 0.41 %. The remainder of the core region refractive index profile is a straight line joining the last point of the xcex1-profile and the core to clad interface point ro on the horizontal axis. The distance from centerline to core to clad interface is in the range 9 xcexcm to 10 xcexcm. Waveguide fibers made in accord with this aspect are predicted to have dispersion slope at 1550 nm in the range 0.065 ps/nm 2-km to 0.067 ps/nm2-km, effective area at 1550 nm in the range 100 xcexcm2 to 105 xcexcm2, and attenuation at 1550 nm in the range 0.182 dB/km to 0.186 dB/km.
A third aspect of the invention is a waveguide fiber having a relative index percent on centerline in the range 0.6% to 0.7%. The radius from the core region centerline to the core to clad interface is in the range 11.5 xcexcm to 12.5 xcexcm. The profile shape is defined by specifying points on the chart of relative refractive index percent versus radius xcex94(r)% versus r. In particular, xcex94(r)% is defined over the radius range 0xe2x89xa6rxe2x89xa65+/xe2x88x920.2 xcexcm by the respective values, xcex94(r)% at r=1+/xe2x88x920.1 xcexcm in the range 0.48% to 0.5%, xcex94(r)% at r=2+/xe2x88x920.1 xcexcm in the range 0.35% to 0.37%, xcex94(r)% at r=3+/xe2x88x920.1 xcexcm in the range 0.24% to 0.26%, xcex94(r)% at r=4+/xe2x88x920.1 xcexcm in the range 0.14% to 0.16%, and xcex94(r)% at r=5+/xe2x88x920.1 xcexcm in the range 0.05 % to 0.08%. The relative index profile is formed by connecting adjacent points by straight lines. For the remainder of the profile, xcex94(r)% is a rounded step index profile in the radius range of 5+/xe2x88x920.1 xcexcmxe2x89xa6rxe2x89xa6ro, and xcex94(r)% at r=5+/xe2x88x920.1 xcexcm is in the range 0.05 % to 0.08%. As is stated above, the relative index percent at the ro point is zero unless stated otherwise. Waveguide fibers made in accord with this aspect of the invention are predicted to have total dispersion slope at 1550 nm in the range 0.066 ps/nm2-km to 0.068 ps/nm2-km, effective area at 1550 nm in the range 80 xcexcm2 to 85 xcexcm2, and attenuation at 1550 nm in the range 0.186 dB/km to 0.190 dB/km.
A fourth aspect of the invention is a waveguide fiber having a relative index percent on centerline in the range 0.40% to 1.05%. The radius from the core region centerline to the core to clad interface is in the range 5.3 xcexcm to 7 xcexcm. The remaining points of the profile, xcex94(r)% for 0 less than r less than ro, are less than or equal to an upper boundary curve JK and greater than or equal to a lower boundary curve LM shown in FIG. 5. The upper and lower boundary curves are selected to provide, at 1310 nm, an effective area greater than or equal to 80 xcexcm2 and an attenuation less than 0.335 dB/km and an attenuation at 1550 nm less than 0.25 dB/km. The attenuation at 1550 nm is preferably less than 0.22 dB/km, and more preferably less than 0.20 dB/km.
The profile shape of this aspect is defined by specifying points on the chart of relative refractive index percent versus radius, i.e., xcex94(r)% versus r. In particular, xcex94(r)% is defined over the radius range 0xe2x89xa6rxe2x89xa66+/xe2x88x920.2 xcexcm by the following values. xcex94(r)% at r=1+/xe2x88x920.2 xcexcm is in the range 0.30% to 0.40%, and the profile shape over the radius range 0 to 1+/xe2x88x920.2 xcexcm is an xcex1-profile having an xcex1 of in the range of 0.8 to 1.2. xcex94(r)% is in the range 0.23% to 0.33% at radius 4+/xe2x88x920.2 xcexcm. xcex94(r)% is a straight line in the radius range of 4+/xe2x88x920.2 xcexcmxe2x89xa6rxe2x89xa65+/xe2x88x920.2 xcexcm and A(r)% is in the range 0.05% to 0.15% at radius 5+/xe2x88x920.2 xcexcm. xcex94(r)% is a straight line in the radius range of 5+/xe2x88x920.2 xcexcmxe2x89xa6rxe2x89xa66+/xe2x88x920.2 xcexcm, and, xcex94(r)% is zero at radius 6+/xe2x88x920.2 xcexcm.
A fifth aspect of the invention is a waveguide fiber preform having a core region and a clad layer, each having respective refractive index profiles. A waveguide fiber is drawn from the preform. The preform refractive index profiles are selected to produce a waveguide having structure and properties in accord with any of the aspects and embodiments set forth above and claimed in any one of claims 1 through 23. The waveguide fiber dimensions scale linearly with the dimensions of the draw preform, so that the geometry of a particular size preform may be readily determined by multiplying the waveguide fiber dimensions by an appropriate constant. This constant depends upon the pre-selected outside diameter of the desired preform.